A valve such as a ball valve has a spherical ball member rotated between open and closed positions relative to a flow passage through the body of the valve. Thus, the sealing surface of the ball member is a spherical sealing surface. The annular seat member contacting the ball member to provide sealing normally is of a generally frusto-conical shape. The balls and seats in ball valves and the swing discs and mating seats in check valves encounter similar problems. It is very difficult to make the seats or the mating areas completely flat and perfectly uniform. It is necessary that one or both of the mating seat members deflect in order to make tight sealing contact. Two methods are presently used to achieve sealing between mating surfaces for such ball valves and check valves. In one method low modulus of elasticity seat materials are used such as plastic materials. The valve bodies are usually made of steel with a modulus of 29 million pounds and the mating contacting seat members are frequently made of plastic with a modulus of less than one million pounds. The low modulus materials readily deform against the sealing surface to make sealing contact.
In another method the seat members are sometimes made of materials having a high modulus of elasticity and are then lapped together to achieve a necessary line contact which provides for a perfect seal. Such lapping is achieved between mating flat surfaces or mating spherical or conical surfaces. Since it is nearly impossible to make two separate pieces which will fit in such close contact, lapping the mating sealing surfaces together is usually the only choice.
There are certain disadvantages in the two methods set forth above. For example, if low modulus seats are used, they are generally made of plastic materials which may fail at high temperatures. Also, the plastic materials deflect enough to make a seal but under conditions of even higher stress may continue to deflect or distort. Often this results in greater than desired lateral movement of the seal. Plastic materials with low modulus are generally also relatively high wear materials and therefore wear, erosion or abrasion of the seals is a major problem. High modulus seats lapped together are extremely expensive to manufacture. Also, high modulus seats lapped together at one temperature will frequently distort at some other temperature, resulting in a slight separation and a loss of perfect seal contact.